typescript 打字稿铸造对象的属性
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Typescript casting object's property
提问by Eonasdan
I'm working with indexeddb and typescript. My issue is that TS doesn't seem to be able handle the event.target.resultproperty. Case in point:
我正在使用 indexeddb 和 typescript。我的问题是 TS 似乎无法处理该event.target.result属性。案例：
request.onsuccess = (event) => {
namespace.db = event.target.result; //the property 'results' does not
//exist on the value of type 'EventTarget'
var a = event.target;
var b = <IDBOpenDBRequest>a;
var c = b.result; // <-- magically there's a results property here
version = parseInt(namespace.db.version);
console.log("version: " + version);
deferred.resolve();
}
So my question is: Is there an easier way to cast the targetproperty to <IDBOpenDBRequest>other then the a, bmethod above?
所以我的问题是：有没有更简单的方法可以将target属性转换为<IDBOpenDBRequest>其他方法而不是上面的a,b方法？
回答by Dick van den Brink
If you are looking for a oneliner you can cast it by adding some extra parenthesis like so:
如果您正在寻找 oneliner，您可以通过添加一些额外的括号来投射它，如下所示：
indexedDB.open("test", 1).onsuccess = (ev) => {
var result: IDBDatabase = (<IDBOpenDBRequest>ev.target).result;
}
Also notice the : IDBDatabasebecause result is typed as anyin the Typescript definition file. It isn't needed but using it as an "any" type would mean no typechecking by the compiler.
还要注意: IDBDatabase因为结果是any在 Typescript 定义文件中键入的。它不是必需的，但将其用作“任何”类型将意味着编译器不会进行类型检查。
Now you can use the result like you want with the methods available as defined here: http://www.w3.org/TR/IndexedDB/#database-interface
现在，您可以使用此处定义的可用方法，根据需要使用结果：http: //www.w3.org/TR/IndexedDB/#database-interface
